1. Field of the Invention
The present invention relates to a method of crystallizing a semiconductor film and an apparatus of crystallizing a semiconductor film and, more particularly, to a crystallizing method and crystallizing apparatus for forming a crystallized semiconductor film by irradiating a non-single-crystal semiconductor film such as an amorphous silicon film with a pulse laser beam.
2. Description of the Related Art
In a liquid crystal display device (LCD), an organic electroluminescence (organic EL) display device, and the like, a thin film transistor (TFT) is formed as a switching element for each pixel, and TFTs are also used in a driving circuit. When the performance of the TFT improves, e.g., when the mobility increases, the threshold voltage decreases, and the characteristic variation reduces, it is possible to obtain effects of, e.g., increasing the operating speed of the circuit, reducing the power consumption, and improving the image quality. A polysilicon TFT is widely used because its mobility is higher than that of amorphous silicon.
A practical method of forming a thin polysilicon film is excimer laser annealing (ELA), which melts amorphous silicon by irradiating it with an excimer laser beam, thereby crystallizing the irradiated region. In this method, the temperature of a substrate to be processed rises to the melting temperature of amorphous silicon. Since, however, this heating temperature is instantaneous in a narrow region of the substrate, amorphous silicon can be crystallized without thermally damaging the substrate to be processed. This makes it possible to use an inexpensive glass substrate as the substrate to be processed, and form polysilicon grains having a grain size of about 0.1 to 1 μm.
When a TFT is formed in the thin polysilicon film thus obtained, the channel region of the TFT contains a large number of grain boundaries. Consequently, the mobility becomes about 100 to 200 cm2/Vs which is greatly inferior to that of a MOS transistor formed in single-crystal Si.
The present inventors previously developed the industrial technique of irradiating an amorphous silicon layer with a laser beam, thereby forming large crystal grains capable of forming a channel portion of at least one thin film transistor. Unlike the conventional transistor in which grain boundaries are formed in the channel region, forming a TFT in a single crystal grain has no adverse effect of grain boundaries, and greatly improves the TFT characteristics. As crystallizing methods like this, the present inventors proposed crystallizing methods described in, e.g., W. Yeh and M. Matsumura Jpn. Appl. Phys. Vol. 41(2002)1909 and JP-A 2006-295097.
The former non-patent reference describes a method of crystallizing an amorphous silicon film by irradiating it with a phase-modulated laser beam having a fluence of 0.8 J/cm2 through a SiON/SiO2 cap film or a SiO2 cap film, thereby laterally growing crystal grains parallel to the film.
The latter patent reference describes a method of laterally crystallizing an amorphous silicon film by irradiating it with a phase-modulated laser beam by using, as a cap film, a SiOx film (x is less than 2), which is non-stoichiometric silicon oxide having light absorption characteristics.
Unfortunately, when the light-transmitting SiO2 cap film is used in the method of the non-patent reference, the cap film itself does not generate heat and has an effect of suppressing cooling of the silicon layer, but the effect is unsatisfactory. Consequently, the crystal growth time cannot be prolonged because the temperature suited to crystal growth cannot be maintained. As a result, the grain size of the obtained crystal texture is not large.
Also, when the light-absorbing SiON or SiOx cap film described in the non-patent reference or patent reference is used, the light absorption spectrum changes if the ratio of oxygen atoms to nitrogen atoms or the ratio of silicon atoms to oxygen atoms in the film changes. That is, a slight change in film formation conditions largely changes the absorption characteristics. This causes crystal grain variations between substrates or in the surface of a substrate, and allows easy occurrence of defects such as a defective circuit operation and uneven display.
Furthermore, when an excimer laser is used as a heating light source in either method, the temperature of a silicon film abruptly decreases after irradiation with the laser beam, because the pulse width of the laser beam is small, i.e., the heating time is short. This makes it impossible to greatly increase the crystal growth time.